The present invention relates generally to magnetic media for recording information, and, more particularly, to disc drives with magnetic head assemblies which record information in tracks on thin film discs.
The computer industry continually seeks to reduce the size of computer components and to increase the speed at which computer components operate. To this end, it is desired to reduce the size required to magnetically record bits of information. It is concomitantly important to maintain the integrity of the information as size is decreased, and magnetic storage of information must be virtually 100% error free. The present invention seeks to address these goals in a disc drive.
Disc drives which magnetically record, store and retrieve information on disc-shaped media are widely used in the computer industry. A write transducer is used to record information on the disc, and a read transducer is used to retrieve information from the disc. The reading and writing processes may be performed by a single structure, i.e., a read-write transducer, or alternatively may be performed by separate structures. In either case, the read transducer and the write transducer are generally both located on a single magnetic head assembly.
The disc is rotated at relatively high speed, and the magnetic head assembly is mounted on the end of a support or actuator arm, which positions the head radially on the disc surface. If the actuator arm is held stationary, the magnetic head assembly will pass over a circular path on the disc known as a track, and information can be read from or written to that track. Each concentric track has a unique radius, and reading and writing information from or to a specific track requires the magnetic head to be located above the track. By moving the actuator arm, the magnetic head assembly is moved radially on the disc surface between tracks. Many actuator arms are rotary, wherein the magnetic head assembly is moved between tracks by activating a servomotor which pivots the actuator arm. A linear actuator may alternatively be used to move a magnetic head assembly inward or outward on the disc along a straight line.
Each magnetic head assembly is typically connected to its respective actuator arm by a flexure or "suspension" arm. The suspension arm functions as a bending spring to bias the magnetic head assembly toward the disc surface. The magnetic head assembly includes a portion known as a "slider". As the disc pack rotates at high speed, the aerodynamic properties of the slider cause the magnetic head assembly to "fly" above its respective disc surface. During use of the disc drive, the magnetic head is designed to fly over the disc surface without contacting the disc, although occasional contacts do happen.
Magnetic thin films are a particular type of magnetic media which are commonly used in computer applications. Thin film media typically consist of a layer or film of a magnetic substance deposited over a substrate. The magnetic substance may be a cobalt based alloy, and the substrate may be a nickel-phosphored aluminum or may be silicon or glass based. A relatively non-magnetic underlayer such as chromium may be used between the magnetic film and the substrate.
To enhance the durability of the disc, a protective layer of a very hard material is applied over the cobalt alloy film. A typical protective layer is an overcoat of sputtered amorphous carbon. The overcoat surface is usually lubricated to further reduce wear of the disc due to contact with the magnetic head assembly. Perfluoropolyethers (PFPEs) are currently the lubricant of choice for thin film recording media. The overcoat and lubricant, while not performing a magnetic function, greatly affect the tribology between the disc and the read-write head, and are very useful in resisting wear of the disc surface which might otherwise be caused by the read-write head. The overcoat and lubricant also help the disc to resist corrosion. While the tribology between a slider and a disc is a function of the properties of the substrate and all the deposited layers, the overcoat and the lubricant are of primary importance. The slider structure also greatly affects the tribology, and sliders are usually formed of a fairly hard ceramic. To record information on the disc, the write transducer creates a highly concentrated magnetic field. During writing, the strength of the concentrated magnetic field directly under the write transducer is greater than the coercivity of the recording medium (known as "saturating" the medium), and grains of the recording medium at that location are magnetized with a direction which matches the direction of the applied magnetic field. The grains of the recording medium retain their magnetization after the saturating magnetic field is removed. As the disc rotates, the direction of the writing magnetic field is alternated based on bits of the information being stored, thereby recording a magnetic pattern on the track directly under the write transducer.
Several parameters of the disc drive system are critical for higher storage densities. Higher coercivity and smaller transition size in the magnetic media lead to higher storage densities. The space necessary to record information in magnetic media is dependent upon the size of transitions between oppositely magnetized areas. The transition size narrows as coercivity is increased, because with high coercivity, the medium can resist the transition broadening due to the neighboring fields.
In thin films, crystalline anisotropy is the primary means of magnetization. Grains are more easily magnetized along the plane of the disc because the grains have a preferred crystalline orientation for magnetization lying along the plane. When applied as a thin film, the crystal structure of the magnetic layer depends firstly on the composition of the magnetic layer, but also depends on the deposition conditions and processes. Cobalt based alloys have been sputtered onto substrates with chromium underlayers to produce media with coercivities in the range of 1800-2900 Oersteds. The coercivities and media noise can be affected significantly by optimizing the deposition processes.
Coercivities and media noise can also be affected significantly by the composition and microstructure of the underlayer. The initial grain growth of the magnetic layer is dependent on the underlying grain structure of the underlayer. Chromium underlayers have often been used to foster a microstructure in a Cobalt-based magnetic layer with high coercivity and low noise. Underlayers of other materials have also been tried, such as NiAl, Mo, W, Ti, NiP, CrV and Cr alloyed with other substitional elements. However, only a few of the underlayers actually perform well, and the most successful underlayer has been pure chromium. The chromium underlayer can be deposited with any of several types of consistent crystalline structure, and most often is deposited with a BCC structure. It is believed that the crystalline structure of the underlayer, and particularly a BCC structure of a Cr underlayer, promotes grainto-grain epitaxial growth of the HCP microstructure of cobalt-based thin films, thereby providing a magnetic layer with beneficial magnetic properties.
Another parameter affecting storage density is head to media spacing. A decrease in head to media spacing allows increased storage density. Present flying altitudes of magnetic head assemblies over the disc surface are usually in the 100-500 Angstrom range. Still lower flying altitudes are anticipated in the future. As flying altitudes are decreased however, the tribology between the slider and disc surface becomes more and more important.
Yet another parameter affecting storage density is the width or minimum separation between adjacent tracks on the disc. Track spacing is dependant upon the minimum size of recorded transition, and also is dependant upon cross-talk from adjacent tracks. Each track must be readable by the magnetic head assembly without interference or cross-talk from adjacent tracks. Each track must also be able retain its recorded information without alteration during writing of adjacent tracks. Present track spacing of commercially available discs is in the range of 5,000 to 10,000 tracks per radial inch, e.g., each track has a width of about 2.5 to 5 microns (25,000 to 50,000 Angstroms).
As storage density is increased, noise in the signal output from the media becomes more problematic. Excessive noise must be avoided to reliably maintain integrity of the stored information. One cause of media noise is cross-talk due to the magnetic field created by adjacent locations on the disc. The magnetic field of each magnetized location on the disc is strongest directly over that disc location, but also emanates outwardly from that disc location in all directions. The magnetic field sensed by a read transducer thus includes not only effects from the magnetization of the disc location directly under the read transducer, but also effects from the magnetization of other adjacent locations. When a read transducer is centered over a track, the tracks are adequately spaced for the head to medium spacing, and the magnetic medium supports sharp transitions, then the track directly underneath the read transducer will dominate the sensed magnetic field, and cross-talk will be minimal. If the read transducer is not centered over the track where the information was written, the distance to the track being read is larger and the distance to adjacent tracks is smaller. Accordingly, inaccurate centering will cause an increase in cross-talk. Similarly, as track spacing decreases, adjacent tracks contribute a greater and greater portion to the sensed magnetic field, and cross-talk increases.
The disc drive must be able to differentiate between tracks on the disc and to center the magnetic head over any particular track. Most disc drives use embedded "servo patterns" of recorded information on the disc. The servo patterns are read by the magnetic head assembly to inform the disc drive of track location. Tracks typically include both data sectors and servo patterns. Each servo pattern typically includes radial indexing information, as well as a "servo burst". A servo burst is a centering pattern to precisely position the head over the center of the track. Because of the locational precision needed, writing of servo patterns requires expensive servo-pattern writing equipment and is somewhat of a time consuming process.
One approach to reduce cross-talk while maintaining increased storage density has been to permanently define tracks by injection molding or stamping a track pattern on a plastic substrate disc. The track pattern includes mechanical voids or depressions in the magnetic layer between tracks. The stamped pattern also includes depressions for servo patterns. The magnetic material layer is then applied at a consistent thickness over the entire disc surface.
After the disc is mechanically fabricated, the servo patterns must be magnetically initialized so they may be magnetically sensed. The entire disc is magnetically initialized with a unidirectional DC magnetic bias. The resulting disc produces a difference in signal intensity in the servo patterns between the relatively strong signal received from the protrusions to the significantly weaker signal received from the depressions.
When this type of disc is used, the distance from the magnetic head to magnetic material in the depressions is further than the distance from the magnetic head to magnetic material in the track. The increased distance both reduces the strength of the signal recorded in the depressions and reduces the contribution from the depressions to the magnetic field sensed by the read head. The depressions accordingly create a barrier between tracks to reduce cross-talk, and higher track density can theoretically be achieved. This approach, referred to as a PERM disc, is being marketed by Sony Corp. The depressions used have a depth of about 0.2 microns (2000 Angstroms) and a width of about 0.2 microns, and track densities of 20,000 tracks per inch (e.g, track widths of 1.25 microns) are reported as being possible.
While the depressions stamped in the disc are helpful in increasing track density, they have a detrimental effect on the tribology of the air bearing slider. The slider no longer travels over a smooth surface, causing several mechanical performance drawbacks. The drawbacks include modulation of fly height when encountering servo patterns, fly height perturbations due to topography changes from the track width definition, glide defects from the stamping process, and disc distortion due to lack of rigidity and yield strength of the plastic substrate material. Other methods to increase track density without the mechanical performance penalties of the Perm disc are needed.